1. Field of the Invention
The present invention relates to a system and method for computing the location of a mobile terminal in a wireless communications network, such as an ad-hoc wireless communications network. More particularly, the present invention relates to a system and method for computing the location of a mobile terminal in a wireless communications network, such as an ad-hoc terrestrial wireless communications network, based on estimated distances to a plurality of terrestrial reference terminals using error minimizing techniques, such as Gauss""s postulate.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as xe2x80x9ccellular networksxe2x80x9d, because the network infrastructure is arranged to divide the service area into a plurality of regions called xe2x80x9ccellsxe2x80x9d.
Specifically, a terrestrial cellular network includes a plurality of interconnected base stations that are distributed geographically at designated locations throughout the service area. Each base station includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from user terminals, such as wireless telephones, located in its coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, the transceiver and user terminals transmit and receive the data packets in multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base station to communicate simultaneously with several user terminals in its coverage area.
In recent years, a type of mobile communications network known as an xe2x80x9cad-hocxe2x80x9d network has been developed for use by the military. In this type of network, each user terminal is capable of operating as a base station or router for the other user terminals, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling user terminals to communicate with each other as in a conventional ad-hoc network, further enable the user terminals to access a fixed network and thus communicate with other user terminals, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled xe2x80x9cAd Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networksxe2x80x9d, filed on Jun. 29, 2001, and in U.S. patent application Ser. No. 09/815,157 entitled xe2x80x9cTime Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channelxe2x80x9d, filed on Mar. 22, 2001, the entire content of both of said patent applications being incorporated herein by reference.
In either conventional wireless communications networks, or in ad-hoc wireless communications networks, it may be necessary or desirable for a user terminal to be capable of knowing or determining its geographic location. Different types of location determining services and techniques for wireless communications networks are described in a publication by Nokia which can be found on the Nokia website at xe2x80x9cwww.nokia.com/press/background/pdf/mlbs.pdfxe2x80x9d, the entire contents of which being incorporated herein by reference. In particular, the Nokia document states that location identification services are currently provided in wireless communications networks based on three major technologies. One of these technologies uses cell identification combined with Round Trip Time (RTT), Timing Advance (TA) and Measured Signal level (RX level), Time Difference of Arrival (TDOA) and Angle Of Arrival (AOA) techniques, the details of which can be appreciated by one skilled in the art. A second technology uses cellular signal timing based methods for code division multiple access (CDMA) and wideband code division multiple access (WCDMA). The third technology described in the Nokia document employs Global Positioning System (GPS) techniques.
Another list of methods and techniques currently used in the wireless communications industry for providing location services can be found at xe2x80x9cwww.911dispatch.com/911_file/location_tech.htmlxe2x80x9d, the entire contents of which being incorporated herein by reference. Although the GPS technique is the last technique mentioned in this list, it generally is viewed as being more accurate than all of the other methods. Further details and descriptions of GPS based methods are set forth in a publication by J. J. Spilker Jr. entitled xe2x80x9cSatellite Constellation and Geometric Dilution of Precisionxe2x80x9d in xe2x80x9cGPSxe2x80x94Theory and Applicationsxe2x80x9d, American Institute of Astronautics, Inc., 1996, in a publication by P. Axelrad et al. entitled xe2x80x9cGPS Navigation Algorithmsxe2x80x9d in xe2x80x9cGPSxe2x80x94Theory and Applicationsxe2x80x9d, American Institute of Astronautics, Inc., 1996, in a publication by Bradford W. Parkinson entitled xe2x80x9cGPS Error Analysisxe2x80x9d in xe2x80x9cGPSxe2x80x94Theory and Applicationsxe2x80x9d, American Institute of Astronautics, 1996, and in a publication by N. Ashby et al. xe2x80x9cIntroduction to Relativistic Effects on the Global Positioning Systemxe2x80x9d in xe2x80x9cGPSxe2x80x94Theory and Applicationsxe2x80x9d, American Institute of Astronautics, 1996, the entire contents of each of these publications being incorporated herein by reference.
Despite the fact that the GPS technique has been in use for a considerable duration of time and most of the world""s navigation relies on this technique, the GPS technique is very susceptible to errors in measurement. Therefore, the GPS technique is capable of providing location determination results with very high accuracy only after performing a relative large number of measurements to remove such errors. A description of the shortcomings of GPS is set forth in a document by IMA entitled xe2x80x9cMathematical Challenges in Global Positioning Systems (GPS)xe2x80x9d which can be found at xe2x80x9cwww.ima.umn.edu/gpsxe2x80x9d, the entire contents of this document being incorporated herein by reference. Certain other tests also demonstrate that the GPS technique is unsuitable for terrestrial-based networks.
In addition, other methods and techniques which do not use GPS satellites for determining mobile station locations in a wireless communications network typically require that the signal from the mobile station is received at at least two cell sites that can measure and process the delay between signals arrival, identify the direction of the signal based on xe2x80x9cpath signaturexe2x80x9d and determine the distance between mobile station and the cell towers. In all of these methods, the processing of the information is executed in a designated central processing unit (CPU) which is typically located at a cell tower next to the base station (BTS). Also, most of these methods were designed to comply with E911 requirements without requiring that excessive modifications be made to existing wireless communications systems. Examples of other location determining techniques are set forth in a document by CERNxe2x80x94European Organization for Nuclear Research, which can be found at xe2x80x9crkb.home.cern.ch/rkb/ANI16pp/node98.html#SECTION00098000000000000000xe2x80x9d, in a document by Wendy J Woodbury Straight entitled xe2x80x9cExploring a New Reference Systemxe2x80x9d, which can be found at xe2x80x9cmenstorsoftwareince.com/profile/newref.htmlxe2x80x9d, and in a document entitled xe2x80x9cAn Introduction to SnapTrac Server-Aided GPS Technologyxe2x80x9d, which can be found at xe2x80x9cwww.snaptrack.com/pdf/ion.pdfxe2x80x9d, the entire contents of each of these documents being incorporated herein by reference.
Accordingly, a need exists for an improved system and method for determining the location of a mobile user terminal in a wireless communications network that substantially eliminate the drawbacks associated with existing location determining systems and methods, such as GPS.
An object of the present invention is to provide an improved system and method for computing the location of a mobile terminal in a wireless communications network, such as an ad-hoc wireless communications network.
Another object of the present invention is to provide a system and method for computing the location of a mobile terminal in a wireless communications network, such as an ad-hoc terrestrial wireless communications network, based on estimated distances to a plurality of terrestrial reference terminals using error minimizing techniques.
These and other objects are substantially achieved by providing a system and method for determining the location of a terminal in a wireless communications network. The system and method perform the operation of receiving respective signals from each of a plurality of reference terminals at the terminal, with each respective signal including information representing a location of its respective reference terminal. Specifically, the terminal receives the respective signals from at least four reference terminals, which can be terrestrial terminals. Also, the wireless communications network can be an ad-hoc wireless communications network, with the terminal and reference terminals being adapted to operate in the ad-hoc wireless communications network. The system and method further perform the operations of estimating a respective distance from the terminal to each of the reference terminals based on the respective signals received at the terminal, calculating a respective simulated pattern, such as a sphere or circle, about each of the respective reference terminals based on the respective distance from the terminal to each respective reference terminal and the respective locations of the respective reference terminals, estimating a location at which each of the simulated patterns intersect each other, and identifying the estimated location as representing the location of the terminal. When estimating the respective distances from the terminal to the reference terminals, the system and error can perform error minimizing techniques, such as those in accordance with Gauss""s postulate.